Rachel Carlson is the sister of Juliet Burke, and suffers from an unknown illness. Rachel was being treated with a series of fertility drugs and was used as a guinea pig in Juliet's experiments in the hopes that she would somehow become pregnant. Juliet obtained the drugs from her ex-husband's laboratory.

After taking two pregnancy tests, Rachel determined that the tests were in fact successful, and that she was pregnant. When Juliet decided to accept the position with Mittelos BioScience, she watched as her sister left with Richard Alpert and Ethan Rom to head to her top secret location.

Around the time that she was six months pregnant, Rachel's cancer returned and it was thought that she would die before she gave birth. On the island that Juliet was sent to, Ben informed Juliet that Jacob was going to see to it personally that Rachel was cured, under the condition that Juliet continued her research.

Rachel was in fact cured of her cancer, and eventually gave birth to a boy, which she named Julian, and was playing with her son in a Miami park on the day Oceanic Flight 815 crashed, all the while unaware she was being watched by Richard, who was transmitting a live feed back to the island for Juliet to see.